Photovoltaic cells, also termed "solar cells", are the subject of intensive technical development as a possible alternative to energy sources dependent upon fossil fuels. Solar cells have been used on a large scale by the U.S. space program for powering satellites and other devices used in space.
In the quest for economic mass production of solar cells, amorphous silicon has shown promise, particularly because this material is relatively abundant and appears to be amenable to continuous processes of production. For these and other reasons, amorphous silicon is currently the most common material used in photovoltaic devices.
Each atom of silicon has four outer-orbit or "valence" electrons which can form covalent bonds with adjacent silicon atoms. Atoms of most other elements have more or fewer than four valence electrons. These other elements, when considered relative to silicon, are termed electron "donors" and "acceptors", respectively. Small amounts of certain of these other elements can be controllably added to silicon in a process termed "doping" to form alloys that either have an excess of electrons or an excess of electron vacancies or "holes". A substance with excess electrons is termed an "n-type" material and a substance with excess holes is termed a "p-type" material. Holes act as positive-charge current carriers. When an n-type and a p-type material are joined, a "junction" characterized by a potential barrier is formed at the interface.
The photovoltaic effect results in the generation of electric power when certain materials absorb radiation such as light. If the radiation energy level is sufficient, separated pairs of electrons and holes are created, thereby creating electric power when the electrons and/or holes pass through a junction. The energy of each photon, however, must equal or exceed an energy "bandgap" existing between valence electrons and conduction electrons in order to produce the electron-hole pairs. This energy barrier is termed the "optical bandgap" E.sub.g. Photons having an energy less than E.sub.g make no contribution to the cell output and are partially absorbed as heat. Photons with energy greater than E.sub.g contribute an energy E.sub.g to the cell output by creating an electron-hole pair. Any excess energy over E.sub.g is dissipated as heat. Semiconductors having optical bandgaps between about 1 and 2.3 eV can all be considered solar cell materials. Generally, a number of factors contribute to energy loss in the transformation of light energy to electrical energy in photovoltaic cells; the theoretical maximal efficiency of silicon solar cells is less than about twenty-five percent.
Amorphous silicon-based alloys have received considerable attention for use in photovoltaic devices. Amorphous silicon is particularly amenable to alloying and doping, thus permitting the material to be readily tailored to form both high and low optical bandgap semiconductors. A solar cell employing a single active semiconductor utilizes only a limited portion of the incident solar spectrum. Because excess photon energy is "wasted" as heat, it is of technological importance to find various semiconductor materials with optical bandgaps that differ over a wide range from that of amorphous silicon.
The most promising way of creating semiconductor materials having different optical bandgaps has been by alloying amorphous silicon (designated "a-Si") with various elements. Although amorphous silicon alloys have applications in a variety of devices, their main application remains in tandem photovoltaic cells. Lin et al., Appl. Phys. Lett. 55:386 (1989). To date, alloys of amorphous silicon have been prepared principally by adding Group IVa elements to the silicon, e.g., carbon, germanium, and tin. Girginoudi et al., J. Appl. Phys. 66:354 (1989). For photovoltaic applications, amorphous silicon and alloys thereof are typically "hydrogenated." For example, such alloys with Group IVa elements are designated "a-Si:C:H" for a hydrogenated amorphous silicon-carbon alloy, "a-Si:Ge:H" for a hydrogenated amorphous silicon-germanium alloy, and "a-Si:Sn:H" for a hydrogenated amorphous silicon-tin alloy. Hydrogenated amorphous silicon alloys have also been prepared that incorporate nitrogen, oxygen, and fluorine. Shufflebotham et al., J. Noncryst. Solids 92:183 (1987). Very few ternary alloys have been produced. The Group IVa elements have been favored candidates for alloying with silicon because these elements resemble silicon in chemical properties. Thus, many researchers expected these elements to "substitute" for silicon in the amorphous network, giving rise to defect-free alloys. However, it has been found that, particularly in the amorphous matrix, such factors as disorder-induced strain, preferential hydrogen bonding to certain elements, differences in atomic size, and other factors caused the silicon alloys formed with these elements to have unexpectedly disrupted structures. Morimoto et al., Jpn. J. Appl. Phys. 20:L833 (1981).
Certain hydrogenated alloys have been shown to produce "device-quality" alloys, particularly by the plasma-enhanced chemical vapor deposition (PECVD) method. (The term "device quality" has an unclear definition in the art. As used herein, "device quality" materials at least have a light-to-dark conductivity ratio of 10.sup.4 or greater and a photoconductivity of about 10.sup.-4 S/cm or greater. Other parameters such as defect-state density and Urbach tail energy are also often considered in the art in determinations of whether or not a material is of device quality.) The optical and electrical properties of amorphous silicon alloys can be further altered by "hydrogen dilution" wherein hydrogen gas is controllably incorporated into the amorphous alloy during formation of the alloy. Apparently, hydrogen ions produced in the plasma, having relatively light mass and high speed, effectively etch out weaker bonding on the growth surface of the alloy, thereby facilitating the formation of a denser alloy network. Hydrogen dilution is also believed to improve the alloys by terminating "dangling" bonds in the amorphous silicon atomic network.
A serious problem with many amorphous silicon solar cells is that their power output (i.e., their "conversion efficiency") will degrade when the cells are exposed to light. Staebler and Wronski, Appl. Phys. Lett. 31:292 (1977). This phenomenon, termed "Staebler-Wronski degradation", results in a considerable reduction in the initial device efficiency, wherein it will be reduced by a considerable amount (15 to 30 percent for a single-junction solar cell; 10 to 15 percent for a multi-junction solar cell) when the device is subjected to operational illumination conditions. Thin, multijunction, amorphous silicon-alloy devices have demonstrated less light-induced degradation than single-junction devices of comparable initial efficiencies. However, further increases in stabilized efficiencies are needed.
Since Staebler-Wronski degradation represents a serious shortcoming, particularly of thin-film, amorphous silicon-based photovoltaic devices, much research effort is currently being dedicated to understanding and solving this problem. Degradation of the intrinsic layer ("i-layer") of solar cells having such a layer appears to be the most serious impediment to long-term cell performance. Light is the principal cause of this problem. Various models to "explain" Staebler-Wronski degradation have been proposed:
(a) Fracture of weak Si-Si or Si-H bonds by the energy released from nonradiative carrier (hole-electron) recombinations.
(b) Changes of charged states of dangling bonds due to interactions with photons.
(c) Changes in dopant coordination in alloys containing dopants due to interactions with photons.
All these models are believed to involve the presence of hydrogen in the material. That is, even though hydrogen imparts certain beneficial properties to an amorphous silicon alloy, such as a desirable change in optical bandgap, hydrogen may also contribute to the Staebler-Wronski effect. To date, however, the effect of hydrogen has been only theoretical; the mechanism by which hydrogen imparts this purported detrimental effect is unknown. In any event, research aimed at reducing the Staebler-Wronski effect has heretofore centered on decreasing the hydrogen content, principally by changing deposition methods and conditions. But, because hydrogen is necessary for terminating dangling bonds, hydrogen cannot be decreased without reducing the quality of the amorphous material. Thus, this research has had limited success in yielding amorphous silicon-based semiconductor material that is both high quality and free of the Staebler-Wronski effect.
Thus, there is a need for "device quality" amorphous silicon-based materials that do not exhibit the Staebler-Wronski effect.